Many radiolocation systems utilize multiple transmitters, each transmitting a continuous unique code division multiple access (CDMA) signal. A CDMA receiver generally tracks these signals using a plurality of receiver channels, each normally containing two or more correlators, to determine the range to each of the transmitters. Traditional CDMA receivers continuously correlate against these continuous CDMA signals. Some prior art positioning receivers sequence a single receive channel across a plurality of continuous CDMA signals. The purpose of the sequencing receiver is to reduce the hardware cost and power consumption of the position receiver.
Not all CDMA based radiolocation systems broadcast continuous signals. When the broadcast CDMA positioning signals are all expected to arrive at a position receiver with similar signal power, then continuous CDMA positioning signals are typically used. However, when the broadcast CDMA positioning signals are received with disparate signal powers, the continuous CDMA positioning signals from the higher signal power transmitters distort the signals received from the lower signal power transmitters. To separate the CDMA positioning signals under these conditions, the CDMA positioning signals may be further separated using either a frequency separation, the so-called Frequency Division Multiple Access (FDMA), in addition to the CDMA separation. Alternatively, the CDMA positioning signals may be further separated using a time division separation, the so-called Time Division Multiple Access (TDMA), in addition to the CDMA separation.
Some radiolocation systems transmit CDMA positioning signals on the same frequency in a pulsed time division multiple access (TDMA) scheme to mitigate the so-called near/far problem. CDMA positioning signals have a specific dynamic range that separates two unique continuous CDMA positioning signals, and this dynamic range is determined by the length of the pseudo random number (PRN) code used to generate the CDMA positioning signal. A near/far problem is produced when one or more of the continuous CDMA positioning signals exceed this dynamic range relative to any other CDMA positioning signal and hence a position receiver cannot distinguish between the two CDMA positioning signals. Additionally, if one or more of the continuous CDMA positioning signals exceed this dynamic range, the radio frequency (RF) front-end of the receiver may become saturated. This situation is most commonly brought about when the CDMA signal transmitters broadcast at the same power levels, but are at disparate ranges relative to the position receiver. The disparate ranges result in varying free space signal power loss, as seen at the position receiver, with signal from the near transmitter being observed as stronger than the signal broadcast from a far transmitter, therefore resulting in the term near/far to describe the problem.
The Radio Technical Commission for Maritime (RTCM) defines one common TDMA broadcasting scheme for a CDMA positioning system. The Radio Technical Commission for Maritime (RTCM) broadcasting scheme divides the 1 millisecond period of the Global Positioning System (GPS) Coarse Acquisition (C/A) code into 11 equal TDMA time slots, each time slot 1/11 of a millisecond in duration. During each millisecond, each transmitter occupies a single TDMA time slot within the millisecond. For subsequent millisecond intervals, the sub-millisecond TDMA time slot assignment is changed based on a predetermined pseudorandom sequence. When each transmitter is represented within a specific time period in the TDMA broadcasting scheme, 1 millisecond in this example, this period is termed the TDMA sub-sequence repeat period. The entire Radio Technical Commission for Maritime (RTCM) broadcasting scheme repeats in its entirety every 200 milliseconds, and is termed a full sequence TDMA repeat period.
Signal-to-noise ratios (SNR) are compromised when a position receiver continuously correlates against TDMA positioning signals. When a position receiver continuously correlates against a TDMA positioning signal, a portion of the correlation time includes the desired TDMA positioning signal, known as the on-pulsed time. During the remainder of the correlation time, known as the off-pulsed time, the received positioning signals do not include the desired signal. During these off-pulsed times, the position receiver is correlating against other TDMA positioning signals and noise, not the desired TDMA positioning signal. Continuously correlating during the off-pulsed times increases the noise brought into the position receiver correlation process without increasing the received signal. Because noise is added to the correlation when the desired signal is not present, the SNR is decreased.
CDMA cross-correlation increases when a position receiver continuously correlates against TDMA positioning signals. CDMA cross-correlation is when two or more CDMA positioning signals, within the dynamic range of the position receiver, are mutually coupled within the correlation process due to the limited dynamic range of the CDMA code separation. The result of cross-correlation is a distortion of the desired signal's auto-correlation function. In most continuously transmitting CDMA positioning systems, the CDMA cross-correlation distortion is relatively small compared to the CDMA code separation dynamic range. However, in a TDMA positioning system, correlating during the off-pulsed times increases the cross-correlation above the level that would be expected with a continuous signal. This is due to the position receiver correlating against other TDMA positioning signals during the off-pulsed times.
Multi-channel prior art position receivers are designed to work with continuously transmitted CDMA positioning signals. An example of a continuously transmitted CDMA positioning signal is the Global Positioning System (GPS). Multi-channel prior art GPS receivers continuously correlate against the plurality of continuous GPS positioning signals. Alternative prior art position receivers sequence between the continuous CDMA positioning signals using a single channel architecture to reduce the hardware cost and power consumption of the position receivers. Examples of prior art sequencing receivers are disclosed in U.S. Pat. No. 4,468,793, issued Aug. 28, 1984, titled “Global Positioning System (GPS) Multiplexed Receiver” and U.S. Pat. No. 4,8949,961, issued Jul. 18, 1989, titled “Fast Sequencing Demodulation Method and Apparatus.” These disclosures teach a single channel receiver architecture that sequences between continuous CDMA positioning signals. These sequencing receivers achieve the stated objects of their invention by reducing receiver channels and hence reducing the cost and power consumption of the receiver. However, prior art sequencing receivers do not include a means for adjusting the sequencing pattern to align to a TDMA broadcasting scheme, and therefore have no means to address the cross-correlation or SNR degradation problems described above.
Prior art TDMA communication receivers transmit TDMA communications signals for increased data throughput to user receivers. Examples of these types of systems are disclosed in U.S. Pat. No. 5,875,402, issued Feb. 23, 1999, titled “Time-synchronous Communication System” and U.S. Pat. No. 5,510,797, issued Apr. 23, 1996, titled “Provision of SPS Timing Signals” and U.S. Pat. No. 5,367,524, issued Nov. 22, 1994, titled “Method for Sequential Data Transmission” and U.S. Pat. No. 6,763,241, issued Jul. 13, 2004, titled “Data Communications Synchronization Using GPS Receiver”. These communication systems provide for TDMA synchronization of user receivers utilizing external synchronization techniques, such as those provided by the Global Positioning System (GPS). Using GPS or a similar satellite-based synchronization technique subjects the TDMA communication receivers to the constraints of the satellite system. Further, additional hardware, namely a GPS receiver or equivalent, is required to facilitate the external synchronization procedure.
There is clearly a need for a position receiver that does not require: (a) a plurality of CDMA correlators running continuously to provide measurement data for TDMA positioning signals, (b) to arbitrarily sequence between a set of continuous CDMA signals, (c) to view satellites for TDMA synchronization, and (d) additional hardware, such as Global Navigation Satellite System (GNSS) receivers, that are only used to establish TDMA timing. A position receiver that operates without these constraints is highly desirable. There is also clearly a need for a position receiver that can provide ranging signals free from the deleterious effects of cross-correlation artifacts and low signal-to-noise ratios (SNR) in TDMA location networks. The present invention achieves this desirable goal by transmitting positioning signals in a TDMA broadcasting scheme, chronologically synchronizing a position receiver to the received TDMA positioning signals, and sequentially correlating on received TDMA positioning signals in synchronism with the network TDMA broadcasting scheme.